Posttraumatic osteoarthritis (PTOA) is a relatively common complication that follows an episode anterior cruciate ligament (ACL) rupture. In the US there are over 100,000 ACL ruptures per year, from which 70% occur in physically active subjects under 44 years of age. At 10 to 20 years after ACL rupture the prevalence of PTOA is estimated to be 50-70%, regardless if surgical intervention was performed or not. The failure of surgery to prevent PTOA lead to the hypothesis that the acute changes in the joint following injury triggers the cascade of events leading to PTOA. There is strong need for noninvasive methods that can identify pathological changes in the joint at molecular levels in the acute posttraumatic phase. Magnetic resonance imaging (MRI) has demonstrated potential to detect changes in the biochemical composition of soft tissues, such as articular cartilage. Pathological changes in articular cartilage are considered critical due to its limited ability to repair. During ACL rupture high compressive and shear forces act on the cartilage, which cause loss of proteoglycan (PG) content, chondrocyte death and disruption of the collagen network leading to cartilage fibrillation and cartilage loss. Thus, measurement of collagen and PG is critical to accurately diagnose cartilage damage following ACL rupture and understanding the natural history of PTOA. However, existing imaging biomarkers have failed to optimally assess PG content and collagen architecture. Though a few MRI markers are sensitive to PG content, T2 relaxation time and the magnetization transfer (MT) are the only MRI markers partially sensitive to collagen. To improve the ability to detect changes in PG and collagen our group introduced diffusion tensor imaging (DTI) of articular cartilage. The key idea behind the use of DTI is that PG and collagen have different effects on the motion of water molecules. Studies performed by our group and others have shown that DTI is sensitive to the PG content through mean diffusivity (MD), and that fractional anisotropy (FA) is a biomarker for collagen architecture. The overarching goal of this proposal is to validate and show feasibility of DTI of articular cartilage as a biomarker for structural and compositional changes in the cartilage matrix after ACL rupture. We propose to test the following hypotheses: (1) DTI can detect the injury- like cartilage damage induced by mechanical overloading with accuracy>80%; (2) Cartilage damage in ACL rupture can be detected by an increase in MD and a decrease in FA. We will test these hypotheses with two aims. Aim 1 will provide ex vivo validation on an injury model by mechanical overloading. We will test the value of DTI to detect cartilage damage and the change in mechanical properties after mechanical overloading. Aim 2 will provide feasibility of DTI of articular cartilage to detect changes in articular cartilage of patients after non- contact ACL rupture (n=11) and healthy volunteers (n=11). Successful completion of our proposal will establish DTI of articular cartilage as a new method for the diagnosis of PTOA, with important implication for the clinical evaluation of novel DMOAs. Data from the proposed project will provide the basis for large longitudinal studies.